This tutorial talks and uses a lots of concepts coming from Symfony, eZ Platform, PlatformUI itself or YUI. Here is a list of documentation pages that are worth reading to completely understand what's going on behind the scenes:

From a Symfony point of view, in this tutorial we mainly wrote a controller and with the associated routing configuration, the Controller book page is definitively the most important Symfony related page to read

The whole YUI documentation could be useful when working with PlatformUI, but amongst others things here is a list of the most important pages:

For the very low level foundations, the guides about Attribute and Base (almost everything in PlatformUI is YUI Base object with attributes), EventTarget (custom events) and Plugin (for tweaking almost any PlatformUI components) can be very useful

A large part of the application is about manipulating the DOM and subscribing to DOM events, this is covered in the Node and DOM Events guides.

The PlatformUI Application is based on the YUI App Framework which itself is mainly described in the App Framework, Router and View guides.